The present invention relates generally to coupling various microwave propagating media and, more particularly, to a method and apparatus for coupling a double-ridge waveguide to a microstrip circuit.
Two of the most commonly employed microwave propagation media are waveguides and microstrip circuits. Usually, waveguides are hollow conductive conduits often having a rectangular or circular cross section intended to propagate microwaves between desired points with a minimum of loss. Waveguides often include ridged structures within the waveguides to change the propagation characteristics of the waveguides and adapt them for particular applications. Square, single-ridge, or double-ridge waveguides are preferred for use with various monitoring and test equipment or for long runs between physically separated parts of the system.
Microstrip circuits have a dielectric material separating a ground plane from a signal carrying microstrip line. Use of microstrip circuits is preferred in many design applications because their cost is negligible when compared to the much costlier waveguide "plumbing." Additionally, electrical components are easily coupled to microstrip. Finally, microstrip circuits are light weight and easily printed using conventional printed circuit board technology.
It is often desirable in microwave systems to employ different types of microwave propagating media including waveguide "plumbing" as the media in some parts of the system and microstrip circuits as the media in other parts of the system. Developing a unified circuit design thus requires efficient coupling between the different propagation media utilized in a given system.
It is well known in the art to couple square and single-ridge waveguides to microstrip circuits. Square waveguides are commonly coupled by adding a tapering ridge as disclosed in U.S. Pat. No. 3,969,691. Coupling a single-ridge waveguide to a microstrip circuit is closely parallel in that the ridge in a single-ridge waveguide is brought down towards the opposite wall, such that the gap is approximately the same thickness as the sum of the thickness of the dielectric, ground plane and microstrip line making up the microstrip circuit. Coupling is then obtained by simply inserting the microstrip circuit into the waveguide so that the ridge contacts the microstrip line and the flat opposing wall contacts the ground plane. In addition to a tapered ridge, other coupling approaches include tapered dielectrics, tapered waveguide walls, and combinations of these approaches, see for example for U.S. Pat. No. 2,825,876.
Double-ridge waveguides present a special problem in that they are inherently balanced transmission lines, whereas microstrip lines are inherently unbalanced transmission lines. Hence, it is desirable to develop a technique for coupling double-ridge waveguides to microstrip circuits for systems including both types of propagating media.